Added: search

* I created a SearchStore to maintain the app state of it
* I abstracted a bit the message-list component so it can handle search result as well as messages from a mailbox
* I cheated a bit for the pagination management (it's done server-side) because the indexer doesn't send the number of results (yet). So page numbers and number of results will be incoherent but it works fine (it'll paginate until there is no result left to show)
* I added a new function to the RouterMixin to ease redirections
* For testing purpose, I added a route '/messages/index/' that will index all the messages
Maybe also trim the query to prevent accidentally running a query for " ".
Should we set a min query size ? On one side, looking for "a" will return a lot of results. On the other side, one may want to look for messages containing the Answer, "42".
A minimal query length of 3 characters seems reasonable.